This invention relates generally to asymmetrical alternating current detecting means and more particularly to means for detecting a direct current component of inrush current in an electric power transformer.
When a power transformer is connected for energization to a source of alternating current (a-c), the resulting magnetizing inrush current in its primary windings may contain a component of direct current (d-c) having a magnitude that initially is large and then decays on a time function determined by the apparent induction of the transformer. The existence and size of the d-c component of inrush current will depend on the residual magnetism in the transformer core and on the part of the alternating voltage wave at which the connection to the a-c source happens to be initiated, and in a worst case condition the magnitude of this component during the initial half cycle of fundamental current can be over 50 per cent of the amplitude of the fundamental alternating current. During succeeding cycles of inrush current the transformer comes out of saturation and the d-c component concurrently decreases toward zero, and at the same time the transformer inductance is increasing so that the d-c component persists for a longer period than might be expected from the initially existing conditions.
The d-c component of transformer inrush current is generally undesirable. It can be a problem particularly in the case of power transformers onboard electrically propelled locomotives or other traction vehicles that run on rails or guideways and utilize wayside sources of a-c power. Such transformers are subject to momentary interruptions of power due to phase breaks in the wayside power conductors (e.g., overhead catenaries) or physical bouncing of the vehicles' current collectors (e.g., pantographs). If reenergization of the transformer were to occur at an unfavorable instant when the source voltage wave is in the vicinity of a zero crossing, the resulting d-c component of inrush current, if not constrained, can sometimes interfere with proper operation of certain types of vehicle warning and protective systems that rely on signals transmitted over the wayside power conductors or rails to detect track occupancy. A relatively high and prolonged d-c component of transformer inrush current in the same conductors or rails could transiently appear as a false "track clear" signal to a track occupancy relay if the relay is of a type utilizing direct current detecting means.
Accordingly, the general objective of the present invention is to provide improved means for detecting and attenuating the d-c component of magnetizing inrush current in a power transformer.
Others have previously proposed harmonic current responsive means for detecting the presence of a d-c component of transformer current. This prior art approach is not capable of accurately discriminating between relatively large and small amounts of the d-c component. In many applications there is no need for the detector to operate if the d-c component has insufficient magnitude and duration to cause possible harm, and therefore another object of this invention is the provision, for detecting the d-c component of transformer current, of a system that responds only when the d-c component exceeds a predetermined threshold level.